M. L. Luhman 1, S. Satyapal 2, J. Fischer 1, J. R. Brauher 3, P. E. Clegg 4, J. W. Colbert 5, P. Cox 6, M. A. Greenhouse 2, S. D. Lord 3, M. A. Malkan 7, G. Melnick 8, H. A. Smith 8, G. J. Stacey 9, & S. J. Unger 4
1 Naval Research Laboratory
2 NASA Goddard Space Flight Center
3 California Institute of Technology
4 Queen Mary and Westfield College
5 University of California, Los Angeles
6 Institut d'Astrophysicque Spatiale
7 University of California, Los Angeles
8 Smithsonian Astrophysical Observatory
9 Cornell University
We discuss observations of the far-infrared line emission from
photodissociation regions (PDRs) in a sample of ultraluminous and other
infrared bright (L > 1010-12 L
)
galaxies as observed with
the Infrared Space Observatory (ISO) Long Wavelength Spectrometer (LWS).
We have compiled ISO LWS spectra of the [CII] 158 
m and [OI] 63, 146
m PDR lines in a variety of infrared bright galaxies, allowing us to
perform a comparative PDR study of the galaxies within the class as well
as with other types of galaxies. As examples of sources where both
starbursts and active galactic nuclei together may excite and heat the
surrounding interstellar medium (ISM), the nature of the emission from the
ISM in infrared bright galaxies as compared to that in normal and active
galaxies is of particular interest. Among the ultraluminous galaxies (L
> 1012 L)
in our sample, we find, for example, that the
emission from the far-infrared PDR cooling lines is a much smaller
fraction of the total far-infrared flux as compared to normal and
starburst galaxies. Using the ISO LWS spectra and recent PDR models, we
investigate in detail this result and other observed properties.